1. Field of the Invention
The present invention relates to an apparatus for warming-up a fuel cell, which generates electric power utilizing oxygen contained in the air.
2. Description of Related Arts
A fuel cell (solid macromolecular type fuel cell) has recently drawn considerable attention as a power source of an electric vehicle etc., because of its cleanness and excellent energy efficiency. For example, Japanese Patent Laid-Open No. 7-240220 discloses a fuel cell system which circulates hydrogen and oxygen supplied into the fuel cell to be reused. The oxygen in this fuel cell system is supplied from an oxidant gas-supplying apparatus. The oxygen remaining unused in the fuel cell is returned to the oxygen-supplying line to be reused. The same can be applied to the hydrogen. The hydrogen remaining unused in the fuel cell is returned to the hydrogen-supplying line to be reused. Because highly pure oxygen and hydrogen are supplied from the oxidant gas-supplying apparatus and the hydrogen gas-supplying apparatus, respectively, the oxygen and hydrogen can be circulated to be reused.
On the other hand, a fuel cell system has been known in which the air is taken from the atmosphere, and the oxygen in the air is used in the fuel cell. In such a configuration, if the oxygen is circulated to be reused, the concentration of the oxygen is decreased (the nitrogen concentration is increased), lowering the efficiency of the fuel cell. Accordingly, the exhaust air discharged from the fuel cell is discharged to the atmosphere without circulation.
The fuel cell can generate electric power in an efficient manner at a temperature higher than the atmospheric temperature (from 80-90° C. in the case of the macromolecular type), and cannot generate electric power if the fuel cell is well warmed. Consequently, it is required to rapidly heat (warm-up) the fuel cell to a prescribed temperature at the time of stating the fuel cell. Particularly, when the fuel cell is carried on an electric vehicle, it should be warmed-up more rapidly. Accordingly, in such a type of the fuel cell system that the air is taken from the atmosphere, a heat exchanger is provided to carry out heat exchange between the exhaust air and the supply air.
However, the supply air is heated by imparting the heat from of the exhaust air to the supply air in the heat exchanger. For this reason, there is a problem that the warming-up the fuel cell cannot be carried out in a rapid manner at starting the fuel cell in which the temperature of the exhaust air is low. This problem becomes serious under low temperature conditions such as cold districts and wintertime, because the temperature of the exhaust gas discharged from the fuel cell is much more lower.
Also, in some cases, the temperature of the exhaust gas discharged from the fuel cell is lowered depending upon the operating conditions. In such cases, the temperature of the supply gas supplied into the fuel cell should be higher than a given temperature. However, in some cases, the supply air cannot be sufficiently heated only through the heat exchanger.